1. Field of the Invention
The present invention relates to novel photoconductive toner particles and, more particularly, it relates to novel photoconductive toner particles comprising a photoconductive pigment and an insulating binder.
2. Description of the Prior Art
A method of recording images utilizing photoconductive particles is known. In such a known method, a charged uniform layer of photoconductive particles formed on a conductive support is exposed with an optical image to dissipate the charge at the exposed areas and then only the toner particles which have a reduced electrostatic attractive force to the support are removed from the support using air, mechanical vibration, etc.
In such a system, the photoconductive layer comprises photoconductive toner particles and hence only the contact points of the toner particles contribute to the flow of the photoelectric current in the photoconductive layer. Therefore, in such a system light must penetrate sufficiently into the interior of the toner particle layer so that the resistance of the contact points of the toner particles is reduced sufficiently. This is quite important since the thickness of the toner particle layer must be about 90 g/m.sup.2 from a practical standpoint (corresponding to a thickness of about 40 microns when no spaces are present between the toner particles).
A typical embodiment of the structure of a photoconductive toner meeting the above requirement is composed of a transparent core material having coated thereon a thin photoconductive layer as disclosed in Japanese Pat. No. 12,385/'69. However, the structure of each a photoconductive toner is inconsistent with the desirability of being able to use the photoconductive toner particles repeatedly in practical use.
First, the photoconductive surface of the toner tends to stain and separate from the core. This tendency is particularly remarkable when the surface layer of toner is composed of a mixture of photoconductive zinc oxide and a resin because such a surface layer has poor mechanical strength.
Another difficulty encountering in using the aforesaid toner particles is that the facility of handling the photoconductive toner particles changes during use. That is to say, the photoconductive particles tend to be electrostatically charged during handling due to their insulating property. In using photoconductive toner particles having a structure composed of a core and a photoconductive surface layer, when the surface layers are stripped during use, the triboelectric characteristics of the toner particles changes, which results in the toner particles aggregating readily. Of course, such a difficulty may be overcome by selecting the materials for the core and the surface layer so that each has the same triboelectric characteristics but this gives rise to new or additional difficulties or restrictions in the freedom for selection and combination of the materials forming the photoconductive toner.
Still another disadvantage is concerned with the properties of the photoconductive toner used in the method as disclosed in U.S. patent application Ser. No. 267,754, filed June 30, 1972, now abandoned. In this method a charged toner image is first formed on a conductive support, electrostatic coating is conducted on the image-carrying surface of the support using a powder paint having the same charge as the toner while the toner image retains the charge to prevent the adhesion of the powder paint to the toner image portions due to electrostatic repulsion, and after exposing the entire surface of the support to light, only the photoconductive toner particles are recovered from the support. In this method the toner particles recovered contain the powder paint and hence it becomes important to separate effectively the two components from each other. This problem can be readily solved if it is possible to use a combination of a powder paint and a photoconductive toner where the photoconductive toner has a specific gravity sufficiently smaller than that of the powder paint and where the photoconductive toner can retain a higher potential. More specifically, if the toner particles can retain a higher potential, strong repulsion can be secured on coating the powder paint and if the specific gravity of the toner is small, recovering the photoconductive toner particles only will be facilitated.
On the other hand, the toners disclosed in British Pat. No. 1,165,017 and Japanese Pat. No. 12,385/'68 do not have a small specific gravity. In fact, the specific gravity of the powder paint used in such system is generally about 2, and even if a thinner photoconductive layer is employed for the photoconductive toners using the technique of British Pat. No. 1,165,017 and Japanese Pat. No. 12,385/'68, the specific gravity of the photoconductive toners is about 1.8 to 2.2 in many cases.
Therefore, a further reduction in the specific gravity of the photoconductive toner has been desired.
In general, the photoconductive toner particles prepared by pulverizing a composition for an electrophotographic material showing good characteristics when the composition is used as a continuous photoconductive layer do not give satisfactory characteristics since light does not penetrate sufficiently into the interior of the photoconductive toner layer when such a material is used. Furthermore, since a photoconductive toner itself fulfills the function of forming an image, the photoconductive toner must have a definite density or light-scattering property, which is inconsistent with the aforesaid requirement of light penetrating sufficiently into the interior of the photoconductive toner layer. On considering these points, the above-described material does not result in a practically useful photoconductive toner.
Furthermore, a photoconductive toner composed of a transparent photoconductive material containing therein a sensitizer or a sensitizing dye is deficient in optical density as an image-forming material. When such a transparent colored toner is used for forming an image on an opaque white support, an image having good contrast is obtained but when a dark colored or black support such as a steel or iron plate and a black paper is used, the image of such a transparent toner formed on such a support becomes undiscernible.
On considering the aforesaid points it can be understood that the characteristics of the photoconductive toner particles are considerably different from the characteristics of conventional toner particles used for forming images on a photoconductive sensitive layer.